Strip casting is a new steel forming process by which strips can be directly produced from molten steel without the need to perform a hot rolling process for markedly reducing manufacturing costs, equipment investment, energy consumption, pollutant emission, and so on.
Particularly, in a twin roll strip casting process in which molten steel is supplied to an upper region of casting rolls (the surface of molten steel is termed “a molten steel surface”), the surfaces of the casting rolls are frequently damaged by skulls or foreign substances growing on the molten steel surface, refractory material particles generating from edge dams, skulls growing on lateral sides of the casting rolls, and so on.
If casting rolls are damaged as described above, the quality of strips is directly affected, and thus it is necessary to protect the surfaces of the casting rolls from foreign objects. For example, a technique for protecting casting rolls from foreign objects is disclosed in Korean Patent Application Laid-open Publication No.: 2005-0065061 (entitled “Method for protecting the casting rolls from the entrapment of foreign object in the twin roll strip casting machine,” and published on Jun. 29, 2005).
However, in the Korean patent application (2005-0065061), if foreign objects are attached to casting rolls while a roll gap is controlled in a position control mode, the operation mode is changed from the position control mode to a pressing force control mode so as to control the pressing force of the casting rolls, and after the foreign objects are removed, the operation mode is changed back to the position control mode. However, due to a delay caused by switching between the two modes, it may take a considerable period of time to remove foreign objects, and thus process yields may be decreased.
In the above-described twin roll strip casting process, a solidified strip is drawn out by inserting a leader strip between the (two) casting rolls and drawing the leader strip out from the carting rolls after molten steel is attached to the leader strip. At this time, casting roll cylinders are used to drive the casting rolls at high pressure (for example, by applying a force of about 20 tons) in an early stage of the casting process, to thus stably attach molten steel to the leader strip. In addition, the two casting rolls are controlled for maintaining a uniform gap (roll gap) between the two casting rolls, stabilizing sealing at edge dams, and easily drawing out strips.
However, values output from a roll gap sensor used to measure the gap (roll gap) between the two casting rolls are largely varied with time, and thus if the gap (roll gap) between the two casting rolls is controlled based on the time-varying values, sealing by the edge dams may become unstable. Particularly, if foreign objects such as molten steel skulls are introduced between the two casting rolls, the surfaces of the two casting rolls may be damaged.
For example, a technique related thereto is disclosed in Korean Patent Application Laid-open Publication No.: 2011-0069598 (entitled “an apparatus and method for controlling wedge of strip in twin roll strip casting process,” and published on Jun. 23, 2011).